1. Field of the Invention
This invention relates to a structure of a charge transfer device such as a charge-coupled device, and more particularly to the charge transfer device having such a channel region that its width is reduced along the charge transferring direction.
2. Description of the Prior Art
The charge transfer device is a device transferring charges through a semiconductor channel region. Such charge transfer devices can be classified by the types of channel regions into a surface channel type in which charges are transferred along the surface region and a buried channel type in which charges are transferred through inside the semiconductor body. The channel region of the surface channel type is defined by channel stopper region surrounding the channel region and/or thin oxide film interposed between the semiconductor substrate and transfer electrodes. The channel region of the buried channel type is also defined by the channel stopper region and by the impurity region formed in a semiconductor substrate having the conductivity type opposite to the impurity region. The potential at the channel region generated by the clock pulses applied to the transfer electrodes depends on the channel width. While the generated potential is high at the wide channel region, it is low at the narrow channel region. This dependency might be an affection from the channel stopper region held at a constant reference voltage. If the channel is narrow, the potential change is restrained by the reference voltage.
Therefore, if the width of the channel region is reduced along the charge transfer direction under one transfer electrode, a potential barrier is produced at the width reduced portion. Some charges to be transferred are retrained by the potential barrier, resulting in a deterioration of charge transfer efficiency. This phenomenon equally appears on the surface and buried channel type devices.